An epidemiologic study is proposed to examine the neuropsychological and thyroid effects of exposure to polybrominated diphenyl ethers (PBDEs), a new and emerging chemical of concern in North America, including the Great Lakes. It builds upon an ongoing investigation of polychlorinated biphenyl (PCB) exposure among older residents of communities along the upper Hudson River. Older men and women were selected as the target populations because they are potentially sensitive subpopulations that already are at risk for age-related neurological deficits, a process that may be exacerbated by polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and other neurotoxins. A total of 253 men and women, 55 to 74 years of age, have participated in the ongoing polychlorinated biphenyl (PCB) project. They have been interviewed regarding local fish consumption, occupational and residential histories, and other relevant variables. They have undergone a neuropsychological test battery capable of detecting subtle deficits in cognition, reaction time, fine motor control, olfactory function, and affective state. They also donated a blood sample for the analysis of 39 polychlorinated biphenyl (PCB) congeners. Archived serum is still available for 190 participants. Upon obtaining informed consent, this serum will be analyzed for 13 polybrominated diphenyl ether (PBDE) congeners and T3, free T4, total T4, and TSH. Ambient and indoor air samples will also be obtained for polybrominated diphenyl ether (PBDE) analysis from the residences of a representative subgroup of participants. Using the interview and air data, exposure to polybrominated diphenyl ether (PBDE) from fish consumption, occupation, and inhalation will be assessed and related to serum polybrominated diphenyl ether (PBDE) concentrations. In addition, serum polybrominated diphenyl ether (PBDE) levels will be related to the neurobehavioral test battery and thyroid hormone concentrations both additively and synergistically with polychlorinated biphenyls (PCBs), after adjustment for a comprehensive set of potential confounders. The association between thyroid hormone levels and the neurobehavioral test results will also be assessed. The proposed study will be among the largest to examine how exposure to polybrominated diphenyl ethers (PBDEs) through fish consumption, occupation, and inhalation impacts body burdens, and is the first to investigate the neurotoxic effects of polybrominated diphenyl ethers (PBDEs) in humans. It will accomplish these goals efficiently by utilizing an existing surveillance cohort and building upon data already collected.